Loudspeaker

ABSTRACT

A loudspeaker including a diaphragm of rigid, self-supporting, foamed plastic material clamped at its peripheral portion to a speaker frame, without providing increased compliance at said peripheral portion, to generate sound by bending motion of said diaphragm, said diaphragm having a soft material such as foamed or unfoamed rubberlike material (e.g. natural rubber, synthetic rubber or rubberlike plastic material) applied on at least a part of the peripheral portion at at least one side thereof, said soft material having relatively high specific gravity sufficient to suppress divided vibration of the diaphragm in low frequency sound range.

United States Patent [72] Inventor HiromiSotome 2,863,520 12/1958 Manleyet al. 181/31 Hamamatsu-shi, Shizuoka-ken, Japan 3,247,925 4/1966\Varnaka 181/31 {2]} Appl. No. 821,803 3,308,904 3/1967 Kelly 181/32 1Filed g n 32 FOREIGN PATENTS [45] Patented pt. l 1 Assignee pp Gum SeizeKabushiki Kaisha 626,876 2/1963 Belgium 181/32 Hamamatsu-shi,Shizuoka-ken, Japan Primary Examiner-Stephen .1. TomskyAttorney-Stevens, Davis, Miller & Mosher [54] LOUDSPEAKER C alms rawmgABSTRACT: A loudspeaker including a diaphragm of rigid, [52] 11.8. Cl 1.181/32 lfl ni f d plastic material clamped at its 1 Int. peripheralportion to a peaker frame providing in- H047 7/00 creased compliance atsaid peripheral portion, to generate [50] Field 01 Search l8l/3l.l,Sound by bending motion f said diaphragm Said diaphragm 31, 32 having asoft material such as foamed or unfoamed rubberlike material (e.g.natural rubber, synthetic rubber or rubberlike [56] References citedplastic material) applied on at least a part of the peripheral UNITEDSTATES PATENTS portion at at least one side thereof, said soft materialhaving 2,716,462 8/1955 Brennan 181/32 relatively high specific gravitysufficient to suppress divided 2,815,823 12/1957 Olson et a] 181/32vibration of the diaphragm in low frequency sound range.

PATENTED SEP H97! 315033427 sum 3 or 5 F|G.4 FIG.5

PATENTED SEP 7 l9?! SHEET l; UF 5 FIG.1O

m wt 83 5&501

FREQUENCY Hz) FIG.11

m w 63 5050 x! FREQUENCY HZ) FIG. 16

FIG. 18

LOUDSPEAKER BACKGROUND OFTHE INVENTION The present invention relates toa loudspeaker having a diaphragm which is clamped at its peripheralportion to a speaker frame, without providing increased compliance atthe peripheral area, so that it makes a bending motion at its wholevibrating area to produce sounds, and more particularly it pertains to aflat-plate type loudspeaker having a diaphragm made of rigid,self-supporting, foamed plastic material, such as foamed polystyrene.

FIGS. 1a and 1b show examples ofa conventional cone-type loudspeaker,which includes a diaphragm I made of conical paper, a speaker frame 2, avoice coil 3, a magnetic circuit 4 and a damper 5. Such a cone-typeloudspeaker is designed to produce sound by piston motion of thediaphragm which is formed at its peripheral portion with a flexiblesuspension 6 having increased compliance and fixed to the speaker frameby means ofa gasket 7.

In such a cone-type loudspeaker it has been proposed to coat theflexible suspension 6 with softening paint 8, as shown in FIG. 1a, or toline the suspension 6 with sound-absorbing material 9 such aspolyurethane foam, as shown in FIG. lb, whereby dips in frequencycharacteristics produced in medium frequency sound range due toopposite-phase resonance of the suspension are reduced and reflection atthe suspension produced in the high-frequency sound range is alsoreduced.

FIGS. 2a and 2b show examples of a flat-plate type loudspeaker having asubstantially flat plate of rigid, self-supporting, foamed plasticmaterial such as polystyrene foam which is fixed at its peripheralportion to a speaker frame, so that it makes a bending motion at itsvibrating part to produce sounds. This type of loudspeaker comprises adiaphragm 11 made of foamed plastic material, a speaker frame 12, avoice coil 13, a magnetic circuit 14 and a damper 15. In general, theflat-plate type loudspeaker is directly clamped at its peripheralportion to the speaker frame, without providing increased compliancearea at its peripheral portion, as shown in FIG. 2a. Such a loudspeakeris designed to provide a large surface area of the diaphragm so that itcan satisfactorily reproduce lowfrequency sound.

In order to reproduce lower frequency sound by smaller surface area ofthe diaphragm, it has been proposed to form .a peripheral portion 16having reduced thickness, as shown in FIG. 2b.

In principle, the effect to be obtained by providing such a thinperipheral portion 16 is substantially similar to that obtained by theflexible suspension of the cone-type loudspeaker as mentioned above.That is, the diaphragm tends to make a piston motion rather than a.bending motion in the lowfrequency sound range, and consequently thethin peripheral portion 16 is likely to 'produce opposite-phaseresonance, in the same manner as in the flexible suspension of thecone-type loudspeaker, in the medium frequency sound range. In order toavoid such a disadvantage, it has been proposed to apply lightsound-absorbing felt 17 or damping material such as liquid rubber to thethin portion 17, which can improve the frequency characteristics to somedegree by reducing the occurrence of the opposite-phase resonance andalso absorbing the vibration of the peripheral area in the medium andhighfrequency sound range.

It will be understood from the above explanation that in theconventional loudspeaker having a peripheral portion of increasedcompliance, such as shown in FIGS. la, lb and 2b, means should beprovided to damp and absorb the vibration of the said portion, in orderto avoid the adverse effect of the vibration of the peripheral portion.In the loudspeaker of the kind as shown in FIG. 2a, in which thediaphragm is directly clamped to the speaker frame without providingincreased compliance at its peripheral portion, the diaphragm is adaptedto make bending motion over the whole area thereof and includes noportion corresponding to the flexible suspension in the cone-typeloudspeaker, so that there is no problem of oppositephase resonance.However, the diaphragm of this kind acts over its whole area like thesuspension portion of the cone-type loudspeaker, so that turbulence infrequency characteristic occurs at a lot of frequency points whereresonant divided vibrations of the diaphragm occur, like the turbulenceproduced by the opposite-phase resonance of the suspension portion. Theterm resonant divided vibration" as used in the specification meanssecond or higher order mode of the resonant vibration. In general, it isvery difficult to decrease such a turbulence in the frequencycharacteristic. In order to improve the frequency characteristic it hasbeen proposed to form the diaphragm in asymmetric form as seen in frontview so that the vibration mode may be divided in complicated formthereby avoiding the occurrence of large peaks and dips in frequencycharacteristic. It has been also proposed to form the central portion ofthe diaphragm in a conical form in order to increase flexural rigidityat said portion, thereby improving the frequency characteristics.

In the loudspeaker having rnultiresonance characteristic as mentionedabove, the peaks and dips can be reduced, but there still remainsappreciable peaks and dips in some range of frequency characteristics.The peaks and dips can be further reduced by making the diaphragm frommaterial having relatively high damping property (material having highinternal loss) or applying damping agent onto the surface of thediaphragm. However such a measure necessarily results in increase ofweight of the diaphragm and considerable decrease of radiationefficiency. Furthermore, even if the internal loss of the material ofthe diaphragm is increased, the peaks and dips produced in the frequencyrange having long wavelength cannot be decreased.

SUMMARY OF THE INVENTION It is an object of the present invention toprovide a loudspeaker of the above kind which suppresses excessiveresonant divided vibration of the diaphragm, thereby decreasing thepeaks and dips in frequency characteristics and obtaining relativelyflat frequency characteristic curve.

It is another object of the invention to provide a loud speaker of theabove kind which particularly reduces dips produced in the low-frequencysound range.

It is another object of the present invention to provide a loudspeakerof the above kind which can obtain a flat frequency characteristiccurve, without reducing radiation efficiency, and reproduce welldampedand clear sounds.

It is a further object of the present invention to provide a loudspeakerof the above kind which can obtain the above effects in simple, stableand efficient manner.

In accordance with the present invention there is provided a loudspeakerincluding a diaphragm of rigid, self-supporting, foamed plastic material(e.g. foamed polystyrene) clamped at its peripheral portion to a speakerframe, without providing increased compliance at said peripheralportion, to generate sound by bending motion of said diaphragm, saiddiaphragm having a soft material such as foamed or unfoamed rubberlikematerial (e.g. natural rubber, synthetic rubber such as neoprene foam orrubberlike plastic material) applied on at least a part of theperipheral portion at at least one side thereof, said soft materialhaving relatively high specific gravity sufficient to suppress resonantdivided vibration of the diaphragm in low-frequency sound range.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. I and 2 illustrate examples ofloudspeakers according to prior art, in which:

FIGS. la and lb are sectional views of a cone-type loudspeaker, and

FIGS. 2a and 2b are sectional views of a flat-plate type loudspeaker.

FIGS. 3a-c to 18 illustrate several embodiments of the presentinvention, in which:

FIG. 3a is a sectional view showing an embodiment of the presentinvention;

FIGS. 3b and 3c are fragmentary sectional views showing alternate formsof the embodiment of FIG. 3a.

FIG. 4 is a rear view of FIG. 3;

FIGS. 5, 6 and 7 are rear views showing modifications, respectively;

FIG. 8 is a sectional view taken along line VllI-VIII in FIG.

FIG. 9 is a sectional view showing another embodiment of the presentinvention;

FIG. 10 is a graph showing frequency characteristic of the flat-platetype loudspeaker of prior art;

FIG. 11 is a graph showing frequency characteristic of the loudspeakeraccording to the present invention;

FIG. 12 is a fragmentary sectional view of another embodiment of thepresent invention;

FIG. 13 is a fragmentary sectional view, similar to FIG. 12, showing afurther embodiment of the invention;

FIG. 14 is a rear view showing another embodiment of the invention;

FIG. 15 is a perspective view, in enlarged scale, showing a peripheralportion of the diaphragm shown in FIG. 14;

FIG. 16 is a rear view, similar to FIG. 14, showing another embodimentof the invention;

FIG. 17 is a sectional view showing another embodiment of the invention;and

FIG. 18 is a sectional view showing the soft material in the embodimentshown in FIG. 17.

PREFERRED EMBODIMENTS OF THE INVENTION FIGS. 3a-l8 illustrate severalembodiments of the present invention.

Referring to FIGS. 3a and 4, the loudspeaker comprises a diaphragm 21made of rigid, self-supporting, foamed plastic material such as foamedpolystyrene, a speaker frame 22, a voice coil 23, a magnetic circuit 24and a damper 25. The diaphragm 21 is fixed at its peripheral portion tothe speaker frame 22 by means of a thin cushion tape 26. A soft material27 having a relatively high specific gravity sufficient to suppressdivided vibration of the diaphragm in low-frequency sound range isapplied on the rear side of the diaphragm around the whole peripheralportion of the vibrating part of the diaphragm. The soft material 27 ispreferably made of the above-mentioned foamed or unfoamed rubberlikeelastomer, and the soft material having adhesive agent previouslyapplied on one surface thereof is most preferred.

It should be understood that the soft material 27 used in the presentinvention must have a sufficiently high specific gravity and that thespecific gravity of the liquid rubber or like damping agent, felt or lowmolecular-weight polyurethane foam (such as moltprene, specific gravity0.025) used in the loudspeaker shown in FIGS. la, lb and 2b is too lowto accomplish the object of the present invention. Accordingly, thespecific gravity of thesoft material should be sufficiently higher thanthat of the diaphragm to suppress the divided vibration in thelow-frequency sound range.

The position, thickness or width of the soft material 27 is selected,depending on shape and size of the loudspeaker and materialand thicknessof the diaphragm. The test has shown that the frequency characteristiccan be considerably improved by applying the soft material on the wholeperipheral portion of the diaphragm, as shown in FIG. 4. However, thesoft material 27 may be applied onto a part of the peripheral portion ofthe diaphragm. It may be applied onto the rear side and/or front side ofthe diaphragm. Even when the soft material is applied as shown in FIG.4, the thickness and the width should be carefully selected and if it istoo small the sufficient effect cannot be accomplished, while if it istoo large, deep dips tend to occur.

By applying the soft material on the peripheral portion of the vibratingpart, the adverse effect produced by the reflection at the fixedperipheral portion of the vibration in the medium and high-frequencysound range can be decreased, without decreasing radiation efficiency ofthe loudspeaker. If the soft material were applied on the centralportion rather than the peripheral portion, more effect could beobtained to reduce peaks and dips due to resonance of divided vibrationsin the low-frequency sound range, but there would be considerabledecrease in radiation efficiency, which is of course disadvantageous.Therefore the soft material should be applied on the peripheral portionof the diaphragm so that the decrease in radiation efficiency isnegligible.

A satisfactory result could be obtained by the loudspeaker as shown inFIGS. 30 and 4, with the following specifications:

Diaphragm (21) material foamed polystyrene specific gravity 0.07

durometer hardness (ASTM designation: D224064T about 50 A-type durometerhardness) size minor diameter 320 mm. major diameter 440 mm.

thickness about 2 mm.

Soft material (27) material foamed neoprene specific gravity 0.20

durometer hardness (ASTM designation; D2240-64T less than 3 A-typedurometer hardness) thickness 5 mm.

width about 30 mm.

Preferably the total weight of the soft material is selected to about20-200 percent of the total weight of the diaphragm and in general theformer is selected to be approximately the same as the latter. In theabove example, the diaphragm having specific gravity 0.020.20 and thesoft material having specific gravity about 0. l0l .25 may be used, andthe diaphragm having thickness l-IO mm. may be used. A further sheet,such as vinyl chloride sheet may be applied on the front surface of thediaphragm to improve its mechanical properties, such as flexuralrigidity and internal friction loss.

In the embodiment shown in FIGS. 3a and 4, the soft material has auniform sectional shape throughout the whole peripheral portion of thediaphragm, but it may partly vary its thickness and/or width or it maybe applied on the diaphragm, with small space being left inside of theinner periphery of the speaker frame.

FIGS. 5-9 illustrate several modifications. FIG. 5 shows a diaphragm 31of asymmetric shape having a soft material 37 which has wider width inthe direction of the major radius and narrower width in the direction ofthe minor radius.

FIG. 6 shows a diaphragm 41 having a soft material 47 applied thereon,with small space 49 being left inside of the inner periphery of thespeaker frame 42.

FIGS. 7 and 8 show a diaphragm 51 having a soft material 57 applied onthe peripheral portion thereof and a weight-adding material 58 made offoamed or unfoamed rubberlike elastomer such as natural rubber orsynthetic rubber, superim' posed on the soft material 57 to furtherincrease the weight thereof.

In these embodiments the soft material may be applied on the frontsurface rather than the rear surface of the diaphragm as shown in FIG.3b wherein like parts are designated by the same numerals as in FIG. 3aor may be applied on both surfaces as shown in FIG. 3c wherein likeparts are designated by the same numerals as in FIGS. 3a and 3b In thelatter case, the soft material may be applied in different manner on thefront side and the rear side.

FIG. 9 shows a diaphragm 61 which is fixed at its peripheral portion toa speaker frame 62 by means of a soft material 67 itself which acts alsoas an adhesive tape.

It will be understood that in accordance with the present invention thefrequency characteristic can be considerably improved by addition of thesoft material having high specific gravity, on the following reasons:(I) When producing sound by bending motion of a diaphragm, in afirst-order (flexural, but not divided) vibration mode, the vibrationmainly occurs in the vicinity of the central portion of the diaphragm,so that the bending angle of the diaphragm at the peripheral portionthereof is relatively small, but in the lower-order mode of dividedvibration the amplitude is relatively large since the divided vibrationof lower order occurs in the lower frequency sound range and the maximumamplitude is located closer to the peripheral portion, so that thebending angle of the diaphragm at the peripheral portion is relativelylarge. In divided vibration of higher order, the amplitude is smallerand the vibration predominantly occurs in the central area, so that thebending angle is smaller. According to the present invention thematerial having relatively heavy weight attached to the peripheralportion of the diaphragm serves to damp the divided vibration of lowerorder, and thus reduce the peaks and dips in the frequencycharacteristic within the low-frequency sound range. (2) The materialapplied to the peripheral portion is sufficiently soft that the flexuralrigidity at the peripheral portion of the diaphragm is not substantiallyincreased thereby even if the soft material is applied to the wholeperipheral portion of the diaphragm. Consequently, lowest resonancefrequency f0 is not substantially affected by the soft material. (3)

The soft material is applied to the periphery of the diaphragm, andtherefore the effective mass of the diaphragm is not substantiallyincreased by the addition of the soft material even if it has relativelyhigh specific rigidity. Accordingly the radiation efficiency is notsubstantially lowered.

By applying the foamed or unfoamed rubber like elastomer on theperipheral portion of the diaphragm, the peaks and dips owing to theresonance at the lower frequency having long wavelength is damped by theweight of said elastomer, while the peaks and dips produced in themedium and high frequency range are absorbed by the elastomer and thereflection at the fixed periphery of the diaphragm is reduced, withoutthe reduction of the efficiency.

FIG. shows a frequency characteristic of the loudspeaker which isessentially identical with that shown in FIGS. 3 and 4 but has no softmaterial applied on the diaphragm. FIG. 11 shows a frequencycharacteristic of the loudspeaker which is identical with the above andhas the soft material applied on the diaphragm as shown in FIGS. 3 and4.

FIG. 12 shows another embodiment of the invention including a diaphragm7i, a speaker frame 72 and a soft material 77. The soft material 77 isformed as a supporting plate which is fixed to the speaker frame 72 andhas an inner periphery projecting toward the center of the loudspeaker,to which the diaphragm 71 is fixed at its periphery. As shown in FIG.12, the outer peripheral surface 73 is substantially aligned with theinner periphery 74 of the diaphragm attaching surface 75 of the speakerframe 72.

In the construction of the diaphragm in which the diaphragm extends tothe outer periphery of the diaphragm attaching surface of he speakerframe, flexural rigidity of the peripheral portion of the diaphragm isrelatively high and consequently lowest resonance frequency f0 is ofrelatively high value. In the construction as shown in FIG. 12, in whichthe outer periphery 73 of the diaphragm terminates at the innerperiphery 74 of the attaching surface 75 of the speaker fame and isfixed through the soft material 77 to the attaching surface 75, theflexural rigidity at the peripheral portion of the diaphragm isrelatively low and consequently the lowest resonance frequency isdecreased, whereby the characteristic in the low-frequency sound rangeis considerably improved.

It should be understood that the outer periphery of the diaphragm isaligned with the inner periphery of the speaker frame, so that thediaphragm cannot make piston motion and the force applied to thediaphragm acts to produce bending motion of the diaphragm which is fixedat its peripheral portion to the speaker frame,

FiG. 13 shows an embodiment of the invention including a diaphragm 31, aspeaker frame 82 and a soft material 87. The soft material 37 is formedas a supporting plate which is similar to the supporting plate 77 shownin FIG. 12. In this form, the outer periphery 83 of the diaphragm 81extends beyond the inner periphery 84 of the attaching surface 85 of thespeaker frame 82 and terminates at the midway of said surface 85. Anadhesive retaining member 86 made of soft foamed plastic material havingdurometer hardness (ASTM designation: D240-64T A-type durometerhardness) of less than 3, such as foamed polyurethane is adhered ontothe peripheral portion of the diaphragm and the soft material 87 so asto hold the peripheral portion of the diaphragm. Thus the diaphragm canbe tightly held on the attaching surface of the speaker frame.

FIG. 14 and FIG. 15 show an embodiment which includes a diaphragm 91, aspeaker frame 92, a magnetic circuit 94 and a soft material 97 made ofrubberlike elastomer, such as natural rubber, synthetic rubber such aschloroprene, rubberlikc plastic or foam of such material or the like.The soft material 97 has an inner periphery 95 in serrated form. FIG. 16shows a modification in which the soft material 97 has an innerperiphery 9% in sinuous form.

In the construction shown in FIGS. 14 and 15 or FIG. 16, the innerperiphery of the soft material 97 is formed in serrated or sinuous shapeand consequently propagating waves reflected at the inner periphery ofthe soft material irregularly change their phases and directions so thatstanding waves due to reflection substantially disappear. Thus thereflection as a whole at the edge of the soft material is decreased andthe peaks and dips in the frequency characteristic owing to theinterference of the propagating waves in the medium and high frequencysound range are decreased, whereby the frequency characteristic in themedium and high-frequency sound range is considerably improved. The softmaterial of this form can be manufactured in effective manner by forminga straight strip of the soft material and then dividing it into twopieces by cutting a serrated or sinuous slit along the centerline ofsaid strip.

FIGS. 17 and 18 show an embodiment including a diaphragm I01, a speakerframe I02 and a soft material 103. The soft material 103 has a taperedinside surface 104 and gradually decreases its thickness toward thecenter of the diaphragm. In such a construction, the boundary conditionat the peripheral portion of the diaphragm, to which the tapered softmaterial is attached, varies gradually and there is not formed awell-defined reflecting line. Accordingly the whole reflection at thesoft material is reduced, whereby the frequen cy characteristic in themedium and high frequency sound range is improved.

While it will be apparent that the preferred embodiments of theinvention disclosed are well calculated to fulfill the objects abovestated, it will be appreciated that the invention is susceptible tomodification, variation and change without departing from the properscope of the appended claims.

Iclaim:

I. A loudspeaker comprising a speaker frame having an attaching surface,a diaphragm of rigid, self-supporting, foamed plastic material withinsaid frame, said diaphragm having a vibrating portion for generatingsound by bending movement thereof, and a soft material having opposingadhesive surfaces fixed at its one surface to the outer edge and part ofthe vibrating portion of the diaphragm and at its other surface to theattaching surface of the speaker frame, said soft material havingrelatively high specific gravity sufficient to suppress resonant dividedvibration of the diaphragm in the low frequency sound range.

2. A loudspeaker according to claim I in which the soft material whichis fixed to the speaker frame has an inner portion projecting inwardlyof said speaker frame and the diaphragm has an outer edge aligned withthe inner edge of the attaching surface of said speaker frame.

3. A loudspeaker according to claim 1 in which the soft material whichis fixed to the speaker frame has an inner por tion projecting inwardlyof said speaker frame, the diaphragm has an outer edge terminating atabout the middle of the attaching surface of said speaker frame and anadhesive retainmaterial has an inner periphery in serrated form. inmember is attached to said diaphragm and to said soft 6 A loudspeakeraccording to claim 1 in which the soft material at the outer portionsthereof. material has an inner periphery in sinuous form.

4 A loudspeaker a o din t l i 1 i hi h h 7. A loudspeaker according toclaim l,in which the the soft diaphr gm has an uter ge aligned with theouter ed f 5 material has a tapered inside surface and graduallydecreases the attaching Surface f the Speaker f its thickness toward thecenter of the diaphragm.

5. A loudspeaker according to claim 1 in which the soft

1. A loudspeaker comprising a speaker frame having an attaching surface,a diaphragm of rigid, self-supporting, foamed plastic material withinsaid frame, said diaphragm having a vibrating portion for generatingsound by bending movement thereof, and a soft material having opposingadhesive surfaces fixed at its one surface to the outer edge and part ofthe vibrating portion of the diaphragm and at its other surface to theattaching surface of the speaker frame, said soft material havingrelatively high specific gravity sufficient to suppress resonant dividedvibration of the diaphragm in the low frequency sound range.
 2. Aloudspeaker according to claim 1 in which the soft material which isfixed to the speaker frame has an inner portion projecting inwardly ofsaid speaker frame and the diaphragm has an outer edge aligned with theinner edge of the attaching surface of said speaker frame.
 3. Aloudspeaker according to claim 1 in which the soft material which isfixed to the speaker frame has an inner portion projecting inwardly ofsaid speaker frame, the diaphragm has an outer edge terminating at aboutthe middle of the attaching surface of said speaker frame and anadhesive retaining member is attached to said diaphragm and to said softmaterial at the outer portions thereof.
 4. A loudspeaker according toclaim 1 in which the diaphragm has an outer edge aligned with the outeredge of the attaching surface of the speaker frame.
 5. A loudspeakeraccording to claim 1 in which the soft material has an inner peripheryin serrated form.
 6. A loudspeaker according to claim 1 in which thesoft material has an inner periphery in sinuous form.
 7. A loudspeakeraccording to claim 1, in which the the soft material has a taperedinside surface and gradually decreases its thickness toward the centerof the diaphragm.